Research and development of Resistive Random Access Memory (ReRAM) are in progress in recent years as a new memory that becomes a successor candidate of floating gate type NAND flash memory. While the NAND flash memory memorizes information by the number of electrons stored in a floating gate electrode, the ReRAM memorizes information by utilizing the changes in resistance of the recording material. The ReRAM is free from problems accompanied with the refining of MOSFET, including the short-channel effect which raises a problem in memory cell of the NAND flash memory and the proximity effect caused by interaction of accumulated charges. Therefore, the ReRAM is expected as a memory more suitable for refining the device than the conventional NAND flash memory.
In many cases, cell array in the ReRAM has a three-dimensional cross point structure prepared by stacking alternately a word line interconnect layer composed of pluralities of word lines extending in one direction and a bit line interconnect layer composed of pluralities of bit lines extending in another direction, by positioning a pillar between each word line and each bit line, and then by joining a variable resistance element and a selector element in series in each of the pillars. The structure realizes a large capacity non-volatile semiconductor memory device. A selector element is provided in order to prevent current from flowing through the not-selected cell array while the current flows through the selected array.
When a variable resistance element conducts monopolar action, it is only necessary to allow current to flow through the selected array in a forward direction, and also to prohibit current from flowing through the non-selected array in a reverse direction. In that case, it is only necessary for the selector element to be a diode having sufficient rectifying performance; for example, it is only necessary to use a PIN diode or an NIP diode.
When, however, the variable resistance element is caused to perform monopolar action, the load on the variable resistance element becomes heavy because the flowing direction of current is the same in both writing and erasing, which raises a problem of the decrease in the number of write-erase cycles on the variable resistance element. Consequently, the variable resistance element is required to perform the bipolar action. During the bipolar action, the applied current is in an opposite direction from each other in writing and erasing, and thus the load on the variable resistance element is decreased and the number of write-erase cycles can be increased.
When the variable resistance element is caused to perform bipolar action, set voltage or reset voltage is applied to the selected cell arrays in reverse directions from each other, while about half the set voltage or the reset voltage is applied to the non-selected cell arrays. Therefore, the selector element is required to be an element which allows not much current to flow therethrough when the applied voltage is less than a certain value, and which allows large current to flow therethrough when the voltage exceeds the certain value.